Electrode regulating system for arc melting furnaces



June 1, 1965 ELECTRODE REGULATING SYSTEM FOR ARC MELTING FURNACES J. B. MURTLAND, JR. ETAL Original Filed April 2, 1965 6 Sheets-Sheet 1 Fig. l.

Fl 36 38 40 r ELECTRODE MOTOR 42 DRIVE CONTROL Q A l 64 so J 58 HASH SENSING :/L'. FILTER AND 1 CONTROL CIRCUIT To Pump :30

24 ARC 44 VOLTAGE 45 1: 5: CONTROL OM T v REFERENCE VOLTAGE 2o 22 \52 Water I In Flg. 2. (I)

l LIJ g 24-- 3 22- ARC GAP LENGTH- INCHES INVENTORS James B.Murflund Jr.,Chcrles F. Rebhun and Harold S. Jackson June 1, 1965 J. a. MURTLAND, JR., ETAL 3,187,073

ELECTRODE REGULATING SYSTEM FOR ARC MELTING FURNACES Original Filed April 2, 1965 6 Sheets-Sheet 2 Fig. 3.

TIME

-- --h- TIME -SECONDS Fig. 5.

l HASH CONTROL.-)L 68 7O PRESSURE MICRONS TIME INVENTORS James B. Murtlond Jn, Charles F. Rebhun and Harold S. Jackson mmv June 1, 1965 J. B. MURTLAND, JR., ETAL 3,187,078

ELECTRODE REGULATING SYSTEM FOR ARC MELTING FURNACES Original Filed April 2, 1963 ARC VOLTAGE-V 6 Sheets-Sheet 3 Fig. 6A.

we I I I ARC LENGTH DECREASING Fig. 6B.

ARC LENGTH- DECREASING Fig. 6C.

ARC LENGTH- DECREASING INVENTORS James B. Murflond Jn, Charles F. Rebhun and Harold S. Jackson ATTORNEY 7 June 1, 1965 J. B. MURTLAND, JR.. ETAL 3,

ELECTRODE REGULATING SYSTEM FOR ARC MELTING FURNACES Original Filed April 2, 1963 6 Sheets-Sheet 4 Fig. 2

40 ELECTRODE MOTOR DRIVE 38 CONTROL as I VOLTAGE SENSING r32 CIRCUIT I A! 2 58 84 V HLTER COUNTER 44 so ARC VOLTAGE -30 CONTROL I2 I 52 REFERENCE VOLTAGE Fig. 8.

40 f ELECTRODE MOTOR DRIVE I CONTROL lOO VOLTAGE SENSING 9 CIRCUIT 8 V FILTER INTEGRATOR ARC VOLTAGE CONTROL INVENTORS James B. Mu'rflond JL, Charles F. Rebhun ,qnd Harold S. Jackson TORNEY June 1, 1965 J. a. MURTLAND, JR., ETAL 3,

ELECTRODE REGULATING SYSTEM FOR ARC MELTING FURNACES 6 Sheets-Sheet 6 Original Filed April 2, 1963 COUNTER 2-60 COUNTER CONTROL CONTROL TIMER l CONTROL TIMER 2 ELECTRODE DRIVE ARC CONTROL 46 SYSTEM ZIOA ZIO

PULSE SHAPER Fig. ll.

United States Patent Oflice 3,187,078 ELECTRODE REGULATIN G SYSTEM FOR ARC MELTING FURNACES James B. Murtland, Jr., and Charles F. Rebhun, Natrona Heights, Pa., and Harold S. Jackson, Troy, N.Y., assignors to Allegheny Ludlurn Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Continuation of application Ser. No. 270,087, Apr. 2,

1963. This application Oct. 8, 1964, Ser. No. 405,646 19 Claims. (Cl. 13-13) This invention relates to electrode regulators for are melting furnaces, and more particularly to a control system for a consumable electrode arc melting furnace adapted to maintain the electrode of such a furnace in predetermined relationship with respect to a molten metal pool beneath it.

This application is a continuation of copending application Serial No. 270,087, filed April 2, 1963, and assigned to the assignee of the present application. The metallurgical furnace control method described herein is claimed in copending application Serial No. 270,088, filed April 2, 1963 and assigned to the assignee of the present application.

Consumable arc melting furnaces are now well known and usually comprise an electrode of the metal to be melted extending downwardly into a mold or crucible which receives the molten metal and within which an ingot is formed. The electrode is connected to one terminal of a direct current voltage source, and means are provided for electrically connecting the other terminal of the source to the mold and, hence, to the molten metal. Usually in starting the melt, a small supply of chips or the like are placed in the mold so that when the arc is struck such chips are melted to form an initial molten metal supply in the crucible mold; and as the arc is maintained between the electrode to be melted and the molten pool of metal beneath it, the end of the electrode is melted due to the heat of the arc. As the electrode is melted, it is deposited in and forms the aforesaid molten pool, the bottom portion of which continuously solidifies as the electrode melts to form an ingot which increases in length, starting from the bottom upwardly. In this process impurities float to and on the top of the molten pool; and assuming that the pool does not solidify during the formation of the ingot, the major portion of the impurities will be excluded from the main body of the ingot.

Such furnaces are usually employed to melt high quality stainless steels, other high quality alloy steels, or reactive metals such as titanium, zirconium and their alloys. The melting process in such a furnace is usually carried out under vacuum or inert atmospheric conditions, the reason being that the presence of air will cause the forma tion of oxides which contaminate the resulting product. The grain structure and general quality requirements of the resulting ingot are most stringent. In this respect, it is oftentimes necessary or desirable to produce a resulting ingot having a substantially uni-form grain structure throughout as well as to substantially completely eliminate inclusions, segregation, stringers and the like.

In the past, one of the most troublesome problems in the production of high quality ingots by the consumable electrode method has been the occurrence of freckles which comprise dark spots or specks in the microstructure of the ingot. Although the reasons for occurrence of such freckles are not entirely appreciated, it is believed that they may be due to minute particles of undissolved metal or non-metallic inclusions.

One of the major contributors to discontinuities and generally poor quality in the as-cast grain structure of an ingot produced in a consumable electrode arc melting furnace is are instability which is a function of not only the distance of the electrode from the molten material,

3,187,078 Patented June 1, 1965 but also the pressure within the crucible in which the ingot is formed. Actually, there are many factors about the characteristics of the are which are not completely understood; however, a short gap is usually considered desirable to minimize the tendency of the arc to form between the sides of the electrode and the mold walls and to concentrate the heat of the arc in the metal bath. Under normal conditions, the arc is anchored at and moves over the surface of the tip of the electrode, and it is expected that this is the preferred location of the are because the cathode normally prefers a hot surface for thermionic emission. However, under certain conditions, the cathode tends to form a multitude of small spots which run up the electrode, these small spots comprising the instantaneous formation of the are between the sides of the electrode and the mold wall. At first these spots do not have any material etfect on are operation; however, if the factors influencing this tendency to form multiple spots continue to act, the arc may become completely unstable. Under this condition, the are completely escapes from the tip of the electrode for a considerable length of time. While this condition exists, melting is not accomplished, reduced heat is being delivered to the ingot, and on small ingots the surface of the molten pool may actually completely freeze over under the influence of such an unstable arc during normal melting with the result that the impurities in the molten pool may be included in the cast ingot. Other undesirable characteristics of an unstable are are cold shuts (i.e., interruption of the arc) and secondary pipe occurring in the as-cast grain structure.

The are gap is usually in the range of about two inches or less, and it has been the practice to attempt to control the position of the electrode relative to the molten pool to maintain a more or less fixed arc gap, the theory being that with a fixed arc gap, the melting procedure will be more or less uniform to produce the desired characteristics in the resulting ingot. In the past, most systems for maintaining the arc gap have relied upon arc voltage (i.e., the voltage gradient across the gap) for regulating purposes. That is, the arc voltage, or approximation thereof, was detected and this voltage used to drive a motor which raised or lowered the electrode, as the case may be, to maintain the arc gap. Apparatus for regulating the position of the electrode, which relies upon the arc voltage for regulating purposes, is not completely satisfactory since there is only a small change of arc voltage with are gap length in the range of gap lengths normally used. Furthermore, as was mentioned above, the voltage drop across the arc is influenced by variables other than the length of the are such as the pressure within the mold which may change abruptly upon the liberation of gases in the melting process. In addition, the arc voltage usually cannot be measured directly, and when the control voltage is derived from terminals connected to the electrode and the mold, the control voltage is aifected by the voltage drops through the contacts, both of which may vary during the melting of the metal. Consequently, if the electrode is positioned so that a constant voltage is maintained between the electrode support and the molten metal, the length of the arc gap is not necessarily or consistently within the desired range. Still another difficulty experienced in attempting to control the position of the electrode as a function of arc voltage is complete or partial shorting of the arc. This condition is usually associated with are instability; and although all of the factors contributing to shorting are not entirely understood, it is apparent that this condition makes the arc voltage control method something less than satisfactory.

The present invention involves a totally unique system for maintaining the arc gap of a consumable electrode furnace in predetermined relationship with respect to the aforesaid molten pool on top of the ingot. In contrast to prior art systems where an attempt was made to regulate the arc gap simply as a function of arc gap voltage with all of its attendant disadvantages, the present invention regulates. the arc gap as a function of an electrical quantity previously unknown. Specifically, it has been found that superimposed on the base arc gap voltage are voltage discontinuities in the form of pulses, each of which represents a momentary increase in the impedance across the gap and persists for a short time such as milliseconds. characteristically, these arc voltage discontinuities occur in groups or bunches at a frequency or repetition rate of below about 30 cycles per second which is the minimum frequency of any ripple content in the direct current voltage. Above a certain arc voltage (i.e., wide arc gap) and immediately after some are disturbances such as a short, the voltage discontinuities do not occur; however, as the arc length is decreased, the discontinuities appear at what is believed to be optimum operating conditions. Surprisingly enough, and in contrast to what might be expected, these voltage discontinuities may be used in a servo system for controlling the arc gap.

Thus, it has been found that the voltage across the arc actually consists of two components. The first component may be termed the base voltage which is comprised of an anode voltage drop, plasma voltage drop and the cathode voltage drop. On an instantaneous basis, the change in this base voltage with a change in arc length is usually referred to as the voltage gradient of the arc plasma. The second component of arc voltage, for want of a better name, may be termed hash which comprises the voltage discontinuities mentioned above. In the specification which follows, these discontinuities, which result from momentary increases in the impedance across the gap, are referred to as hash pulses. It has been found that the presence or absence of these pulses has a substantial effect upon the melt rate, with very little change in apparent input power to the furnace. When the hash pulses are present during melting, the melt rate is found to be higher. It is thought that this is caused probably by out-gassing of the metals during melting with a resulting increase of pressure in the melting chamber; and it is possible that the hash is an indication of a confinement of the arc to an area between the electrode and the molten pool such that radiation to the surrounding crucible walls is minimized and the heat flow out of the melt area held to a minimum. Where melting is accomplished in the absence of the hash condition and at a substantially constant arc voltage, a relatively uniform melt rate is also obtained but at a much lower rate than that obtained with the hash present. Furthermore, with hash present, the melt rate can be increased without an appreciable increase in power input; and under certain conditions power input actually appears to decrease with an increased melt rate when hash is present. Thus, it can be theorized that the presence of hash indicates optimum operating conditions.

Although the foregoing discussion has been more or less limited to the voltage discontinuities resulting from momentary increases in the impedance across the arc, it should be understood that the impedance increases give rise to other fluctuations in an electrical characteristic of the arc gap, such as current discontinuities, which can be used for purposes of control. The current discontinuities, for example, comprise decreases in are cur rent which can be used in a control system in the same manner as the hash pulses. Thus, periodic increases in the impedance of the arc will occur in synchronism with the occurrence of each increasing voltage pulse and decreasing current pulse.

As an overall object, the present invention seeks to provide apparatus for controlling the arc gap in a consumable electrode furnace to attain optimum operating conditions and to produce a resulting product of exceptional quality.

More specifically, an object of the invention is to provide a control system of the type described utilizing the voltage discontinuities or other fluctuations mentioned above as a control parameter.

Still other objects of the invention are to provide an arc gap control system for a consumable electrode furnace wherein short circuits are substantially reduced, a more uniform and faster melt rate is achieved, greater pressure stability within the furnace is attained, and greater are stability is effected.

In accordance with the invention, the control system includes means for detecting fluctuations in an electrical characteristic of the arc gap which recur beneath a predetermined frequency, means for producing an electrical signal which varies in response to variations in the fluctuations which recur beneath said predetermined frequency, and apparatus for controlling the position of the electrode of the furnace with respect to the molten pool beneath it as a function of said electrical signal. In the preferred embodiment of the invention, the means for detecting electrical fluctuations which recur beneath said predetermined frequency comprises a filter which eliminates the ripple content of the rectifiers supplying the direct current voltage across the gap and leaves only those fluctuations (i.e., hash) occurring within a predetermined frequency band. However, any device other than a conventional filter may be used which selectively attenuates the ripple content and passes the hash. In a specific control arrangement, wherein a filter is utilized to filter out frequencies above thirty cycles per second, voltage fluctuations occurring at a frequency beneath thirty cycles per second are counted and the electrode lowered by an amount which increases as the number of fluctuations counted for a fixed interval of time decreases, the reason being that as the hash drops off, the arc length approaches a point where it is too great. In another embodiment of the invention, the hash is integrated and the integrated voltage used to drive the electrode downwardly toward the molten pool. The invention, however, is not limited to counting the discontinuities or integrating the discontinuities, as it will be appreciated that other arrangements can be used such as controlling the electrode position as a function of the pulse height of the hash content, pulse height above a threshold value, or pulse width and rise times, any one of which could be used with circuitry required for effecting a measure of the arc voltage for control purposes and resulting control to predetermined conditions.

As was mentioned above, the hash or the discontinuities are not present continually in the arc voltage, but only when the arc gap is within a more or less optimum range. Therefore, it is desirable to incorporate into the control system means for sensing a predetermined high are voltage level as well as a predetermined low arc voltage level and to switch the control system back to arc voltage control when these voltage limits are exceeded. As will be understood, such a provision is necessary in order to facilitate initiation of the are at the beginning of an electrode melting process when the hash does not occur, and also to facilitate lowering the electrode under conditions where the arc gap is greater than that at which the hash occurs.

The above and other objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification and in which:

FIGURE 1 is a schematic diagram of a consumable electrode furnace including a block diagram of the hash control system of the present invention as well as the conventional arc voltage control system;

FIGURE 2 is a graph illustrating a typical voltage gradient across the arc gap in a consumable electrode i furnace as well as the variation in the gradient with variations in pressure;

FIGURE 3 is an illustration of the arc voltage in a consumable electrode furnace showing the ripple content therein before filtering;

FIGURE 4 is a graph illustrating typical voltage discontinuities comprising the aforesaid hash which occur in the arc voltage at a predetermined frequency;

FIGURE 5 is a graph illustrating the manner in which the pressure within a consumable electrode furnace is stabilized when the hash content of the arc voltage is employed to control electrode position;

FIGURE 6A is a graph illustrating the manner in which the hash occurs only beneath a predetermined arc length;

FIGURE 63 is a graph illustrating the integrated hash content as the arc gap is decreased;

FIGURE 6C is a graph illustrating a curve which occurs when the integrated hash is mixed with true are voltage;

FIGURE 7 is a schematic block diagram of one type of hash control system in accordance with the present invention wherein the hash content is counted;

FIGURE 8 is a schematic circuit diagram of another embodiment of the invention wherein the hash content is integrated and the integrated voltage used to control electrode position;

FIGURE 9 is a block diagram of a further embodiment of the invention wherein the hash content is integrated and the resulting integrated voltage combined with true are voltage to produce a control signal;

FIGURE 10 is a block diagram of still another control system wherein the hash content is filtered from the true are voltage to effect a control signal for regulating purposes;

FIGURE 11 is a detailed schematic circuit diagram of the block diagram of FIGURE 7 wherein the hash content is counted for purposes of control; and

FIGURE 12 is a portion of a schematic circuit diagram which may be incorporated into the system of FIGURE 11, but which utilizes current pulses rather than voltage pulses for purposes of control.

Referring now to the drawings, and particularly to FIG- URE 1, a consumable electrode arc furnace is schematically illustrated and identified by the reference numeral 10. The furnace comprises a conductive mold or crucible 12 which may, for example, be fabricated of copper. Covering the upper open end of the mold 12 is a gastight housing 14 having a connection at 16 to means, not shown, for evacuating the chamber 18 formed by the mold 12 and the housing 14 covering it. Alternatively, the chamber 18 could be filled with an inert gas; however, in either case the metal to be melted is protected from oxidation. Surrounding the mold 12 is a water jacket 20 having inlet and outlet ports 22 and 24 connected thereto.

The mold 12 contains the ingot 26 which is formed from a molten pool 28 adjacent the lower end of an electrode 30 of the metal to be melted. Electrode 30 extends upwardly from the mold 12 and is connected at its upper end to a reciprocable rod or electrode ram carrier 32 which extends through a seal 34 in the housing 14. The ram 32 is connected to a suitable drive which may be either mechanical or hydraulic. In this specific embodiment the electrode drive 36 comprises a rack and pinion or worm and screw arrangement connected through shaft 38 to a drive motor 40, the arrangement being such that as the motor 40 is rotated in one direction, the ram 32 and electrode 30 carried thereby will move upwardly; whereas rotation of the motor 40 in the opposite direction will cause downward movement of the electrode 34). Connected to the ram 32 and, hence, to the electrode 30 is the negative terminal 42 of a direct current voltage source, not shown. Positive terminal 44 of this same voltage source is connected to the mold 12, the arrangement being such that an are 45 will be struck between the lower end of the electrode and the bottom of the mold 112 and where metal is in the mold beween the electrode and the upper end of the ingot 26, thereby forming heat which progressively melts the end of the electrode and causes the formation of the aforesaid molten pool 28. As electrode 30 is melted, it is, of course, necessary to move it downwardly by means of the motor and electrode drive 36 in order to maintain the desired arc gap.

As was mentioned above, it has been the practice to attempt to control the position of the electrode 30 to maintain the desired arc gap by using the arc gap voltage as it appears between the electrode 30 and ingot 26. This control apparatus is schematically illustrated in FIGURE 1 in its simplest form and comprises an arc voltage control circuit 46 connected through leads 48 and 50 to the electrode 30 and the mold 12, respectively. In circuit 46 the arc gap voltage is compared with a reference voltage from source 52; and assuming that switch 54 is closed, the difference voltage is applied to motor control circuit 56 to raise or lower the electrode 30, as the case may be. That is, if the arc voltage should fall, the arc gap is theoretically decreasing. Therefore, the circuit 46 will apply a voltage to motor control circuit 56 to cause motor 4%) to raise the electrode 30. Similarly, if the arc voltage should rise, the arc gap is theoretically widening, and the circuit 46 will apply a signal to motor con trol circuit 56 to cause motor 40 to lower the electrode 30.

As was explained above, the system just described for controlling the arc gap as a function of arc gap voltage is not completely satisfactory since there is only a small change of arc voltage with are gap length in the range of gap lengths normally used. Furthermore, the voltage gradient across the arc is not constant. This is shown, for example, in FIGURE 2 where are voltage is plotted against arc length in inches. It will be noted that over a range of about two inches in arc length, the arc voltage varies only about two volts. Consequently, it is extremely difficult to sense variations in arc voltage as a function of electrode position. Furthermore, the arc voltage may vary not only as a function of arc length, but also as a function of the pressure within chamber 18. Thus, the arc voltage characteristic for a given arc length may be that shown by the full line curve A in FIGURE 2 at one pressure, and that shown by the dotted line curve B at another pressure. Actually, the pressure is constantly varying within the chamber 18 and particularly in the zone of the arc per se due, among other reasons, to the liberation of gases during the melting process. In this connection, it is to be noted that there is no known practical means for measuring pressure in the arc zone in a commercial arrangement, meaning that the pressure cannot be correlated to the voltage. Consequently, it can be readily appreciated that the pressure within chamber 18, and, hence, the arc voltage for a given arc length is not constant. Still another disadvantage of the arc voltage control system is due to the fact that the voltage drop across the contacts to which the leads 48 and 50 are connected may not be constant. This may occur, for example, due to the heating of the contacts and other effects.

The new and improved system of the present invention is schematically illustrated, in its simplest form, by the blocks 58 and 60 in FIGURE 1. In this arrangement, the arc voltage is again sensed; however, it is passed through a filter 58 to eliminate the ripple content in the direct current voltage applied to terminals 42 and 44. As is known, a direct current, other than that derived from a battery or other chemical source, is not absolutely unvarying in its magnitude, but will contain a small ripple Which is due either to the rectifiers employed in rectifying an alternating current voltage or due to the action of a commutator in a direct current generator. The are voltage as it may appear across leads 48 and St? is shown in FIGURE 3; and it will be noted that there is substantial ripple occurring at a frequency of about 360 cycles per second with a peak-to-peak voltage of about one to five volts above and below the median direct current voltage of twenty-four volts. In accordance with the present invention, the direct current voltage illustrated in FIGURE 3 is passed through the filter $8 which eliminates the ripple content therein. In most applications, the filter 58 will be such as to pass only those recurring voltage fluctuations having a frequency beneath about thirty cycles per second. The specific value of thirty cycles per second, however, is not to be considered as limiting, the only requirements being that the filter eliminate the ripple voltage.

It would, of course, be expected that after passing through the filter 58 which eliminates the ripple content of the direct current voltage, a more or less unvarying steady-state direct current voltage would be obtained. Contrary to expectations, however, the output of filter 58 appears as shown in FIGURE 4; and it will be noted that superimposed on the direct current voltage are volt age discontinuities 62 each of which persists for about forty milliseconds. Characteristically, the voltage discontinuities or pulses s2 occur in bunches and comprise the hash discussed above. The exact reason for the occurrence of the voltage discontinuities or hash is unknown; however, the fact is that we have found that they do occur and we use such discontinuities in the present invention for controlling the position of electrode 3d.

Referring again to FIGURE 1, the output of filter 58 comprising a signal corresponding to that as shown in FIGURE 4 is applied to the hash sensing and control circuit on adapted to produce an output signal on lead 64 which varies as a characteristic of the hash content. Assuming that the switch 66 is closed, this signal is applied to the motor control circuit to control the position of electrode 30. As will be understood, only one switch 54 or 66 will be closed at any one time; however, since the hash occurs when the arc voltage is only within certain upper and lower limits of arc length, it will be appreciated that when the gap is outside of these limits, the switch 54 must be closed and are voltage control employed. One specific arrangement for automatically switching from hash control to are control to suit varying operating conditions and are lengths is hereinafter described with reference to FIGURE 11.

The hash sensing and control circuit as preferably counts or integrates the pulses or voltage discontinuities 62 in the signal shown in FIGURE 4. As was mentioned above, however, these arrangements are in no way to be considered as limiting as other possibilities include measuring the widths of the pulses 62, measuring the rise time of the pulses, and other similar arrangements.

With reference to FIGURE 5, the effect of hash control on pressure within the chamber 18 is shown. Between points 68 and 7%) the position of electrode 30 Was controlled as a function of the hash content shown in FIG- URE 4-; and it will be noted that the pressure is much more constant than at other times outside of the range between points 68 and '70 where arc voltage control is employed. This evidences the fact that during hash control, the liberation of gases is much more constant than during arc voltage control and accounts, at least in part, for the fact that during hash control the melting rate is constant and increased for a given input power.

With reference now to FIGURE 6A, it can be seen that above an arc length identified by the line 72, the hash or voltage discontinuities '62 do not occur. Below the arc length 72, however, the hash appears in the voltage gradient with the magnitude of the voltage discontinuities increasing as the voltage decreases. By integrating the hash content shown in FIGURE 6A, the integral fiiat shown in FIGURE 6B increases as the magnitude of the arc voltage decreases. As will be understood, this derivative can be used to control the position of the electrode 349 with respect to the molten pool 28 beneath it. Furthermore, as shown in FIGURE 6C the integrated hash, when mixed with true arc voltage, can result in. a dish-shaped curve 74. If a desired arc voltage identified by the line 76 in FIGURE 6C is selected, it is possible that two balance conditions are possible, identified as points '78 and 8th in FIGURE 6C. If point '78 is, by chance, the point reached first, the system will be stable and will feed the electrode downwardly properly. If, however, the point it'll on curve '74 is reached first, the system will be unstable since the slope of the voltage curve is reversed. If a desired arc voltage as represented by the line 82 is selected, the system will never reach balance and periodic dipping and shorting out will occur.

With reference to FIGURE 7, a control system is shown in which elements corresponding to those shown in FIGURE 1 are identified by like reference numerals. In this case, the output of filter 58 is fed to a counter 34 which will apply a signal to the motor control circuit 56 to lower the electrode 3h when the pulses counted by the counter 84 fall below a predetermined number over a fixed period of time. If the arc voltage between the electrode 36 and molten pool 28 falls below or exceeds the voltage limits between which hash occurs, a voltage sensing circuit 86 will actuate relay 8% to open contacts 99 While closing contacts 92, thereby transferring the control from the hash content to arc voltage. Thus, hash control is effective above and below the aforesaid lower and higher voltage limits, while arc voltage control is in effect at all other times. Under actual operating conditions, however, the necessity of using voltage control will occur only under exceptional circumstances such as a short circuit, the hash control being in operation at all other times.

In FIGURE 8 another control arrangement is shown wherein the output of filter 53 is applied to an integrator 94, the output of the integrator being applied through contacts as of relay 98 of the motor control circuit 56. Whenever the arc length falls below a level at which it intersects the curve 72 shown in FIGURE 6A, the voltage sensing circuit I00 will actuate the relay 98 to open contacts as and close contacts lltlZ to transfer the control from the hash content to the arc voltage.

In FIGURE 9 still another control arrangement is shown wherein the rectifier ripple is filtered from the arc voltage across leads 48 and 50 by means of a low pass filter 104 which is similar to the filter 58 shown in FIGURES 1, 7 and 8, and which is adapted to pass only those frequencies below about thirty cycles per sec ond. After passing through the low pass filter lltl t, the

resulting hash is divided into two channels. The one channel includes a hash filter 1% which removes the hash voltage from the filtered arc voltage leaving the true base arc voltage. The other channel includes a hash integrator 1% which provides a voltage at its output pro portional to the amount of hash. This voltage is fed to a hash content comparator where it is compared with a preset hash reference voltage from source 112. Variations of hash content from the hash reference volt age from source 112 are used to set a reference volt age in circuit IM. The reference voltage from source I12 is then compared to the base voltage of the arc from filter tee in a preamplifier 116. Deviations of the base voltage from the reference voltage from source 112 are amplified in amplifier I18 and used to drive the ram through circuit 120.

Still another control arrangement is shown in FIG- URE 10. In this case, the rectifier ripple is again filtered by a low pass filter 122. The resulting hash voltage at the output of filter 1122 is then passed through a second filter 124 which removes the hash content, leaving only the true base are voltage. This base are voltage is then compared with a reference voltage from source 126 in a preamplifier 128, and any deviations are applied through power amplifier to the ram drive circuit 132.

With reference now to FIGURE 11, a complete detailed circuit diagram for a control system employing the counter arrangement of FIGURE 7 is shown. The are voltage appearing between the electrode 30 and mold 12 is applied through leads 134 and 136 to a filter 138 which removes the ripple, leaving only the hash voltage comprising the voltage discontinuities decsribed above which recur at a frequency below about thirty cycles per second. This hash voltage is adapted to be applied through a pulse shaper 140 and through contacts of relays, hereinafter described, to a pair of counters 142 and 144. The counter 142 may, for example, be

set to count forty pulses representing forty voltage discontinuities in the hash content; while the counter 144 may be designed to count sixty pulses. Whenever the number of pulses to which the counter 142 or 144 is preset are counted, a relay 146 or 148, respectively, will be actuated.

Referring again to the leads 134 and 136 across which the arc voltage is applied, these leads are connected to a primary winding 150 of a first saturable core reactor 152 and also to a primary winding 154 of a second saturable core reactor 156. Also wound about the saturable reactor 152 is a bias Winding 158 having input terminals adapted for connection to a source or bias voltage, not shown. In a similar manner, a bias winding 162 is wound about the saturable core reactor 156 and is adapted for connection through terminals 164 to a source of bias voltage, not shown. The windings 150 and 158 and the bias voltage applied to terminals 160 is such that it opposes that applied to winding 159; and that applied to terminals 164 also opposes to that applied to winding 154. Furthermore, the voltage applied to input terminals 160 is such that it will trigger core 152 when the voltage across input winding 150 (i.e., the arc voltage) falls below the minimum value at which hash control can be used. In a somewhat similar manner, the voltage applied to terminals 164 is such that the core 156 will trigger when the arc voltage exceeds that at which the hash appears, this voltage being represented by the line 72 in FIGURE 6A. Consequently, when the arc voltage falls below the predetermined value at which hash control can be used, a voltage will be induced in output winding 166 of core 152 to actuate relay 168, thereby closing contacts 168A. Likewise, when the voltage rises above the maximum voltage at which hash control can be used, a voltage will be induced in output winding 170 on saturable reactor 155 to energize the relay 172 and close its normally open contacts 172A.

When contacts 168A close, they actuate delay timer 174. Delay timer 174 as well as the other timers hereinafter described, is of the type which will close a pair of contacts 174A after a predetermined period of time has elapsed subsequent to the closing of contacts lfi A. If, however, the contacts 168A are not closed for the aforesaid predetermined period of time to which the timer 174 is set, the timer 174 will then reset itself automatically to begin counting for a full period without closing contacts 174A. Assuming that an under voltage condition occurs and that relay 168 is actuated to close contacts 168A and that these contacts are closed for a sufiicient time to cause the timer 174 to close its contacts 174A, the relay 176 will be energized, thereby closing contacts 176A and 176C while opening contacts 176B. When contacts 176A close, they energize an L0 timer 178, and assuming that the contacts 176A are close for a predetermined period of time, the timer 178 will close its contacts 178A to thereby energize the relay 180. Thus, relay 180 will be energized upon the occurrence of an under voltage condition only upon the expiration of the time delays etfected by timers 174 and ll) 178 and will remain energized only so long as the under voltage exists. That is, when the under voltage condition ceases, contacts 163A, 174A, 176A and 178A will open to break the circuit to relay 180.

With reference, now, to the saturable reactor 156 and its associated relay 172, when contacts 172A are closed, they energize a timer 182, thereby causing its to close its contacts 182A after an amount of time determined by the time delay of timer 182. When contacts 182A close, they energize, through the previously closed contacts 172A, a relay 184. Thus, it can be seen that relay 181) will be energized upon the occurrence of an under voltage condition, but only if that under voltage condi tion persists for a predetermined period of time determined by the timers 174 and 178. Similarly, if an over voltage condition occurs, relay 184 will be energized, but only after a predetermined period of time determined by the time delay of timer 182. As will be seen, the relays 189 and 184 are used to disconnect the drive system for the electrode 30 from hash control and put it back into arc voltage control; however, this will not occur until the under voltage or over voltage condition exists for a specified period of time as was explained above.

Relay 186i is provided with a pair of normally closed contacts lfitiA, while the relay 184 is provided with two pairs of normally closed contacts 184A and 184B. Contacts 1813A and 184A are included in the energizing circuit for a relay 1&6, this relay having a pair of normally closed contacts 186A in the circuit connecting the arc control system 45 to the motor control circuit 56. Thus, whenever the contacts 186A are closed, the system will be operating under are voltage control; whereas Whenever the relay 185 is energized and contacts 186A open, arc voltage control will no longer be in efiect and hash control takes over.

Also included in the energizing circuit for relay 181 are the normally open contacts 188A of a relay 183, the normally closed contacts 190A of a relay 190, and a manually operated switch 1%. Relays 188 and 190 are connected to automatic programming equipment, not shown herein, adapted to energize the relay 188 at the beginning of a melting operation and to energize relay 190 at the completion of the melting operation. Thus, assuming that the switch 192 is manually closed, relay 188 will be energized to complete the circuit to relay 136, assuming that contacts 180A and 184A are not open. Alternatively, relay 136 can be energized with contacts 180A and 184A closed by closing manually operated switch 194 which is connected in shunt with switch 192 and contacts 188A and 193A. In either case, the relay 186 will be energized assuming that neither an over voltage or an under voltage condition exists.

At the start of the melting operation, the electrode 30 will be in contact with the bottom of the mold 12 or starter material therein and the arc voltage will be very low, thereby causing energization of relay 180 to open its contacts 180A. Consequently, at this time, the relay 186 will be deenergized and contacts 186A will be closed, whereupon the normal conventional arc control system 46 will cause the electrode 30 to move upwardly. After the voltage across the arc rises to the point where relay 168 is energized, the relay 180 will become deenergized to close contacts 180A, thereby energizing the relay 18 6 to open contacts 186A. At this point, the arc control system 46 is no longer connected to the motor control circuit 56 and the system is under hash control.

As soon as the contacts 180A close and the system goes into the hash control phase, a count timer 196 will be energized through the normally closed contacts 198A of a relay 198 and normally closed contacts 176B of relay 176 which is now deenergized due to the fact that the arc voltage has now exceeded its predetermined lower voltage limit at which hash control takes effect. At the same time that counter 196 is energized, a relay 200 is lll also energized through contacts 1198A and 17613. Energization of the relay Ztltl causes its normally open contacts ZlltlA to close; and assuming that the contacts 1988 of relay 1% are not open due to energization of the relay 19%, pulses from the pulse shaper 14th will be applied to the counters M2 and 144, whereupon each of the counters will begin its counting process.

After the expiration of the count time of counter 11%, it will close its normally open contacts 1 96A, thereby energizing relay 282 which closes its normally open contacts 202A and EMS. When contacts 2ll2A close, they energize a relay Zlld through normally closed contacts EMA of a relay 2% and normally closed contacts 146A of relay lid-6 at the output of counter 142. The relay Ztl-twill remain energized to close its normally open contacts EMA until relay 2% is energized or relay 146 is energized after the counter 14.2 has counted pulses in the hash signal equal to the preset count of the counter M2.

Referring again to the relay 2G2, when contacts 196A close, it is energized to close its contacts ZtlZA and M23. Contacts 292A provide a holding circuit for the relay 2%. That is, once the contacts 262A are closed, the relay 2632 will remain energized until the contacts Edd l open, regardless of the condition of contacts 196A. When contacts ZtlZB close, a control one timer is energized; and after a predetermined period of time, the control one timer 268 will close its contacts 2813A, thereby energizing the relay 2% to open contacts EMA, thereby breaking the circuit to relay 292. When relay 2% is energized, it also closes contacts 2%3 to energize relay 210; whereupon contacts 2169A close to reset the counters 14-2 and 144.

When contacts EMA on relay Ell close, they energize relay 212 which closes contacts 212A in the motor control circuit 56 to cause the electrode 3G} to move downwardly. Downward movement of the electrode will continue until the circuit to relay 2% is broken and contacts 264A open. Relay 212 can also be energized through contacts 214A of relay 21d and through the normally open contacts ZlldA of a control two timer 2%. Relay 214- may be energized through normally open contacts 196A of the count timer 1%, through normally open contacts 1463 of relay 1 56 and through normally closed contacts lat- A of relay MS. Alternatively, relay 2.14

may be energized through contacts ZtBZA, 296A, 1463 and 143A. It will be readily appreciated that the relay 214 can be energized only when relay 146 is energized and relay 148 is deenergized. This occurs when the pulses in the hash content are at least equal to the count of counter 142 and less than the count of counter lldd.

When relay 214 is energized, the control two timer 2316 is also energized; and after a predetermined period of time it will open its contacts 216A to break the circuit to relay 212. Thus, whenever the relay 212 is energized by closure of contacts 214A, it will remain energized for a period of time determined by the control timer 2%.

If a very high hash count is counted by counters M2 and 144, it means that the electrode 3% is too close to the molten pool beneath it. Hence, as mentioned above, the hash count increases as the arc gap decreases. Under these conditions, the relay 14-8 will be energized to open its contacts 148A, thereby preventing energization of relay 214 and also preventing energization of relay 222 to move the electrode Sail downwardly. Likewise, under these conditions the relay 146 will have been energized to open contacts 14 6A and prevent energization of relay 2%. When the relay res is energized, however, it closes its normally open contacts 1488; and assuming that contacts 184B are closed in the absence of an over arc voltage condition, the relay 218 will be energized to close its contacts 213A, thereby causing the electrode 3b to move upwardly. This action will continue until the contacts 1848 open. As was explained above, the relay 184 will be energized when the maximum desirable arc voltage is reached. Thus, the electrode 39 will move upwardly t2 only until the maximum arc gap is reached; whereupon hash control will again take effect.

In the operation of the device, assuming that the arc voltage is within the maximum and minimum values within which hash control is effective, both of the contacts lldtlA and 184A will be closed. This causes energization of the relay 1% to open contacts 186A and break the circuit between the arc control system 46 and the motor control circuit 56. If at any time are voltage should rise above or fall below the maximum and minimum permissible values respectively, relay res will become deenergized to close contacts 186A and connect the arc control system as to the motor control circuit 56. At the same time, the contacts 1186B will close to energize relay 21b and reset each of the counters 142 and 144.

When hash control takes effect, the count timer 1% will be energized; and at the same time the relay 2% will be energized to close its contacts ZtlllA, thereby applying pulses from the pulse shaper 14th to counters M2 and Md. The function of timer 1% is to determine the period during which pulses are counted by counters 14-2 and 144 before reset. This period will depend upon certain variables, one of which is the diameter of the electrode. That is, as the diameter of the electrode increases, the distribution of the pulses becomes less constant, meaning that the period of timer 1% should be increased as electrode diameter increases and vice versa in order to provide a statistically determinate system. It will be assumed that counter 142 will count forty pulses before energizing relay 146 and that counter 144 will count sixty pulses before energizing relay 148. Let us assume that during the count period of the count timer 1%, forty pulses are not received by the counters 142 and 14d, meaning that neither of the relays 146 nor 143 will be energized. Under these conditions, the hash rate is very low meaning that the electrode 3t should be moved downwardly through a maximum amount. At the expiration of the count time of timer 196 with less than forty pulses counted, the contacts 196A will close, thereby energizing relay 292. When contacts 2923 close, the control one timer 2% is energized. At the same time, since relay 14s is not now energized and contacts ran are closed, the relay 204 will become energized to close its contacts ZMA, thereby energizing the relay 212. This closes contacts 2112A to cause the electrode as to move downwardly. Downward movement of the electrode 3% will continue until the contacts 206A, open. This occurs, as will be understood, upon the expiration of the time period of control timer 208 when contacts EMA close. Thus, when less than forty pulses have been counted the electrode 3% will be driven downwardly by a maximum amount determined by the time interval of control one timer 208.

Let us assume, now, that more than forty pulses have been counted and less than sixty pulses. Under these conditions, the relay 1% will become energized to open contacts 146A and close contacts 14613, but relay 148 will remain deenergized. With contacts 146A now open, the relay 26M. cannot become energized; however, relay 214 will become energized since contacts lid-8A are now closed with relay 148 deenergized. When relay 2M- becomes energized, the contacts 214A close, thereby energizing relay 212 to close contacts 212A and again drive the electrode 3th downwardly. This action will continue until contacts 216A of control two timer 216 open. The control two timer 216 is energized at the same time that relay 214 is energized. Consequently, relay 212 will remain energized for a period of time determined by the control two timer 216, this time being less than the period of control one timer 208.

Now, if more than sixty pulses are counted by the counters 1142, and 144, both of the relays 146 and 148 will be energized. This means that the electrode 3t) is too close to the molten pool beneath it or at the desired spacing. With the relays 146 and 148 both energized,

13 a circuit is now completed to relay 218 through contacts 1468 and 148B. Relay 218 will remain energized as mentioned above until the contacts 184B open upon the establishment of the maximum arc voltage condition, whereupon hash control will again be put into efiect.

The control sequence can be summarized as follows:

(1) Under voltage cndition'.Relay 168 becomes energized, and after a period of time determined by timers 174 and 178, relay 180 will become energized to deenergize relay 186 and close contacts 186A. Consequently, the system now responds to signals from are control system 46.

(2) Over voltage condition-Relay 172 becomes energized, and after a period of time determined by timer 182, contacts 182A close to energize relay 184 and open contacts 184A, thereby again deenergizing relay 186 to close contacts 186A and again cause the motor control circuit 56 to be responsive to signals from the arc control system 46.

(3) At all times other than an over voltage or under voltage condition, relay 186 will be deenergized and system will operate under hash control.

(4) Hash control with count less than forty during period of count timer 196, meaning that electrode 30 should be lowered through a maximum preset amount- Relays 146 and 148 will remain deenergized and relay 212 will be energized for the period of timer 208 to lower electrode 30 for the period of timer 208.

(5) Hash control with count greater than forty and less than sixty.Relay 146 becomes energized, but relay 148 is deenergized to energize relay 212 for the period of timer 216, this period being less than that of timer 208 to drive electrode 30 downwardly, but through a shorter distance than when the count is less than forty.

(6) Hash control with count greater than sixty, meaning that electrode is too close to molten pool-430th of the relays 146 and 148 will be energized to energize relay 218 to cause electrode 30 to move upwardly until relay 184 is energized to signal an over voltage condition, whereupon hash control again becomes effective. Alternatively, however, the relay 218 may be eliminated such I that the electrode 30 will remain stationary until its lower portion has been melted suificiently to again widen the arc gap.

With reference, now, to FIGURE 12, a system is shown which is similar to that of FIGURE 11 except that current discontinuities, rather than voltage discontinuities, are used to effect control. Elements of FIGURE 12 which corerspond to those of FIGURE 11 are identified by like reference numerals. The system is the same as that of FIGURE 11 except that instead of filtering arc voltage, are current is filtered. This is derived by filtering the signal across an impedance, such as resistor 220, in the negative input power lead to the furnace. After filtering in circuit 138, the current discontinuities, which are negative-going as mentioned above, are used in the same manner as the voltage discontinuities of FIGURE 11.

Although the invention has been shown in connection with certain specific embodiments, it will be readily appreciated that various changes in form and arrangement of parts may be made to suit requirements without departing from the spirit and scope of the invention. In this respect, it will be apparent that a larger number of counters may be used in the control system of FIGURE 11 to eflect a finer and more accurate control.

We claim as our invention:

1. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring fluctuations in an electrical characteristic of said are gap, which fluctuations occur as a result of momentary increases in the impedance across the arc gap, said momentary increases in impedance being superimposed on the impedance associated with base are voltage and current, means coupled to said detecting means for producing an electrical signal which varies in response to variations in said fluctuations, and apparatus responsive to said electrical signal for controlling the position of said electrode with respect to said molten pool.

2. The combination of claim 1 wherein said electrical characteristic comprises the voltage across said are gap.

3. The combination of claim 1 wherein said electrical characteristic comprises the current through said are gap.

4. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring fluctuations in an electrical characteristic of said arc gap, which fluctuations occur as a result of momentary increases in the impedance across the arc gap, said momentary increases in impedance being superimposed on the impedance associated with base are voltage and current, means coupled to said detecting means for producing an electrical signal which varies in response to variations in the integral of said fluctuations, and apparatus responsive to said electrical signal for controlling the position of said electrode with respect to said molten pool.

5. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring fluctuations in an electrical characteristic of said gap, which fluctuations occur as a result of momentary increases in the impedance across the arc gap, said momentary increases in impedance being superimposed on the impedance associated with base are voltage and current, means coupled to said detecting means for producing an electrical signal which varies as a function of the frequency of recurrence of said fluctuations, and apparatus responsive to said electrical signal for controlling the position of said electrode with respect to said molten pool.

6. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring fluctuations in an electrical characteristic of said gap, which fluctuations occur as a result of momentary increases in the impedance across the arc gap, means coupled to said detecting means for producing an electrical signal which varies in response to variations in said fluctuations, apparatus responsive to said electrical signal for causing said electrode to move downwardly toward said molten pool, and apparatus responsive to the voltage across said gap and operable when said voltage across the gap falls beneath a predetermined value for causing said electrode to move upwardly away from said molten pool.

7. The combination of claim 6 and including circuitry for stopping the upward movement of said electrode when the voltage across said gap reaches a predetermined maximum value.

8. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for filtering the voltage across said are gap to eliminate all recurring voltage fluctuations other than those recurring beneath a predetermined frequency, means coupled to said filtering means for producing an electrical signal which varies in response to variations in the voltage fluctuations which recur l beneath said predetermined frequency, and apparatus responsive to the output of said last-named means for controlling the position of said electrode with respect to said molten pool as a function of said electrical signal.

9. In a consumable electrode furnace of the type in which an arc is struck by applying a direct current voltage between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring hash voltage fluctuations in the direct current voltage across said gap, which fluctuations comprise momentary increases in the voltage across the arc gap, means for producing an electrical signal which varies in response to variations in said voltage fluctuations, and apparatus for controlling the position of said electrode with respect to said molten pool as a function of said electrical signal.

it). In a consumable electrode furnace of the type in which an arc is struck by applying a direct current voltage between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range comprising means for detecting hash voltage fluctuations in said direct current voltage comprising momentary increases in the voltage across the arc gap and which recur at a frequency beneath about thirty cycles per second, means for producing an electrical signal which varies in response to variations in said voltage fluctuations, and apparatus for controlling the position of said electrode with respect to said molten pool as a function of said electrical signal.

11. In a consumable electrode furnace of the type in which an arc gap isstruck between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining the arc gap between said electrode and said molten pool within a desired range, comprising means for filtering the voltage across said are gap to eliminate all recurring voltage fluctuations other than those recurring beneath a first predetermined frequency, and apparatus for moving said electrode downwardly toward the molten pool when the frequency of said recurring voltage fluctuations drops below a second predetermined frequency which is lower than said first predetermined frequency.

12. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for filtering the voltage across said are gap to eliminate all recurring voltage fluctuations other than those recurring beneath a predetermined frequency, counting means for counting the recurring voltage fluctuations which pass through the filtering means, said counting means being adapted to produce an output electrical signal after a predetermined number of said voltage fluctuations have been counted, means for period-.

ically resetting said counting means after the expiration of a fixed period of time, and motor means actuable in response to said output signal from the counting means for moving said electrode downwardly toward said molten pool.

13. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining the arc gap between said elect-rode and said molten pool within a desired range, comprising means for filtering the voltage across said are gap to eliminate all recurring voltage fluctuations other than thosev recurring beneath a predetermined frequency, means for integrating the voltage fluctuations which recur beneath said predetermined frequency, and apparatus responsive to the integrated output of said in iii tegrating means for controlling the position of said electrode with respect to said molten pool.

lid. In a consumable electrode furnace of the type in which an arc is struck by applying a direct current voltage between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining are are gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring voltage fluctuations in the direct current voltage across said gap, which fluctuations occur as a results of momentary increases in the impedance across the arc gap and which recur at a frequency beneath any ripple content of the. direct current voltage, means for producing an electrical signal which varies in response to variations in the voltage fluctuations which recur beneath said predetermined frequency, apparatus responsive to the voltage across said gap for moving said electrode downwardly toward the molten pool when said voltage across the gap exceeds a first predetermined value, apparatus responsive to the voltage across said gap for moving said electrode upwardly awayfrom the pool when the voltage across the gap falls below a second predetermined value, and apparatus for controlling the position of said electrode with respect .to said molten pool as a function of said electrical signal when the voltage across the gap is etween said first and second predetermined values.

15. In a consumable electrode furnace of the type in which an arc is struck by applying a direct current voltage between a consumable electrode and a molten pool of metal beneath it, the improvement of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring voltage fluctuations in the direct current voltage across said gap, which fluctuations occur as a result of momentary increases in theimpedance across the arc gap and which recur at a frequency beneath any ripple content of the direct current voltage, counting means for counting said voltage fluctuations which recur at a frequency beneath any ripple content of the direct current voltage and adapted to produce an output signal whenever the number of fluctuations counted during a predetermined time interval exceed a fixed amont, apparatus responsive to the voltage across said arc for moving said electrode downwardly toward said pool whenthe voltage across the gap exceeds a first predetermined value, apparatus responsive to the voltage across said are for moving said electrode upwardly away from the molten pool when the voltage across said gap falls beneath a second predetermined value, and apparatus operable when the arc voltage is between said first and second predetermined values and responsive to said electrical signal at the output of said counting means for moving said electrode downwardly toward said molten pool through a predetermined increment.

is. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, and wherein the voltage across the arc gap comprises a base are voltage component and a second component superimposed on the base are voltage component and comprising voltage discontinuities in the. form of pulses which are caused by momentary increases in the impedance across the arc, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring fluctuations in said second component of arc gap voltage, means coupled to said detecting means for producing an electrical si nal which varies in response to variations in said fluctations, and apparatus responsive tosaid electrical signal for controlling the position of said electrode with respect to said molten pool.

17. In a consumable electrode furnace of the type in which an arc is struck between aconsumable electrode:

and a molten pool of metal beneath it, and wherein each of the voltage and current electrical characteristics of the arc gap comprises a base component and a second component superimposed on the base component and comprising electrical discontinuities in the form. of pulses which are caused by momentary increases in the impedance across the arc, the combination of control apparatus for maintaing the arc gap between said electrode and the molten pool within a desired range, comprising means for detecting recurring fluctuations in said sec ond component of at least one of said are gap electrical characteristics, means coupled to said detecting means for producing an electrical signal which varies in response to variations in said fluctuations, and apparatus responsive to said electrical signal for controlling the position of said electrode with respect to said molten pool.

18. In a consumable electrode furnace of the type in which an arc is struck between a consumable electrode and a molten pool of metal beneath it, the combination of control apparatus for maintaining the arc gap between said electrode and the molten pool Within a desired range, comprising means for detecting recurring fluctuations in the hash component of arc gap voltage, which fluctuations occur as a result of momentary increases in the impedance across the arc gap, means coupled to said detecting means for producing an electrical signal which varies in response to variations in said fluctuations, and apparatus responsive to said electrical signal for controlling the position of said electrode with respect to said molten pool.

19. In a consumable electrode furnace of the type in which an arc is struck with a consumable electrode to effect the melting thereof and wherein each of the voltage and current electrical characteristics of the arc gap of said are comp-rises a base component and a second component superimposed on the base component and comprising electrical discontinuities in the form of hash pulses which are caused by momentary increases in the impedance across the arc, the combination of control apparatus for maintaining the arc gap of said are within a desired range, said control apparatus comprising means for detecting recurring fluctuations in said second component of at least one of said are gap electrical characteristics, means coupled to said detecting means for producing an electrical signal which varies in response to variations in said fluctuations, and apparatus responsive to said electrical signal to position said electrode to effect control of the arc gap within said desired range.

References Cited by the Examiner UNITED STATES PATENTS 2,026,943 1/ 36 Kennedy et al 314-73 X 2,027,224 1/36 David 2l9-135 2,798,107 7/57 Boron et al 13-13 2,915,572 12/59 Buehl 13-13 2,942,045 6/60 Johnson l3l3 3,128,364 4/64 Wanttaji et al 31468 X RICHARD M. WOOD, Primary Examiner. JOSEPH V. TRUHE, Exai'niner. 

1. IN A CONSUMABLE ELECTRODE FURNACE OF THE TYPE IN WHICH AN ARC IS STRUCK BETWEEN A CONSUMABLE ELECTRODE AND A MOLTEN POOL OF METAL BENEATH IT, THE COMBINATION OF CONTROL APPARATUS FOR MAINTAINING THE ARC GAP BETWEEN SAID ELECTRODE AND THE MOLTEN POOL WITHIN A DESIRE RANGE, COMPRISING MEANS FOR DETECTING RECURRING FLUCTUATONS IN AN ELECTRICAL CHARACTERISTIC OF SAID ARC GAP, WHICH FLUCTUATIONS OCCUR AS A RESULT OF MOMENTARY INCREASES IN THE IMPEDANCE ACROSS THE ARC GAP, SAID MOMENTARY INCREASES IN IMPEDANCE BEING SUPERIMPOSED ON THE IMPEDANCE ASSOCIATED WITH BASE ARC VOLTAGBE AND CURRENT, MEANS COUPLED TO SAID DETECTING MEANS FOR PRODUCING AN ELECTRICAL SIGNAL WHICH VARIES IN RESPONSE TO VARIATIONS IN SAID FLUCTUATIONS, AND APPARATUS RESPONSIVE TO SAID ELECTRICAL SIGNAL FOR CONTROLLING THE POSITION OF SAID ELECTRODE WITH RESPECT TO SAID MOLTEN POOL. 